1. Field of the Invention
This invention relates to an optical projection imaging system, and more particularly to a system for very high-speed imaging of high-resolution patterns for the production of high-performance electronic products with large areas, such as printed circuit boards, multichip modules, displays and semiconductor integrated circuits.
2. Description of Related Art
Economical manufacturing of many electronic and opto-electronic products requires fabrication of millions of microscopic structures on a single large substrate. The structures can be active devices, such as transistors in a flat-panel display (FPD) of a semiconductor integrated circuit (IC), or passive patterns such as interconnecting conductors in a printed circuit board (PCB) or a multichip module (MCM). The large substrate can be a board, a display panel or a silicon wafer. The pattern sizes in these diverse products range from sub-micron for semiconductor ICs to multi-microns for displays, MCMs and PCBs. The substrate size requirements vary from a few square inches for small modules to a few square feet for large PCBs and displays.
A critical and common factor in the above applications is that they all require a large-area patterning system that can provide the required resolution over the entire substrate. Cost-effective production requirements also prefer systems with high processing throughputs. The patterning technology selected determines not only the ultimate performance of the product, but also the economics of the entire manufacturing process through such key factors as throughput and yield. No patterning system currently exists that meets these criteria satisfactorily. The disclosed invention describes a system technology that delivers all of the desired performance and cost features in a patterning system, namely, large-area capability, high resolution, high throughput, and high yield.
There are three primary types of patterning systems currently in wide use:
contact printing systems, PA0 projection imaging systems, and PA0 focused-beam laser direct writing systems.
Contact printing suffers from two inherent disadvantages: generation of defects during patterning, and mask life degradation. A representative contact printer for volume manufacturing consists of a fixture to align and hold the substrate in contact with the mask, and a collimated high-intensity light source to flash expose the mask pattern onto the substrate. Such systems sometimes feature a two-drawer substrate handling facility, allowing the user to load a second substrate while a first is being exposed; double-sided drawers may allow exposure of opposite substrate sides simultaneously or sequentially. Most contact printers use mercury-xenon or metal-halide lamps, with powers ranging from 2 to 8 kW.
Conventional single-field projection imaging systems eliminate the disadvantages of contact printing resulting from defects and wear, but are limited in the largest substrate size they can expose due to their small image field. Step-and-repeat projection systems overcome this constraint by partitioning the substrate into several segments; however, this decreases throughput and creates the difficult requirement of precisely stitching the segments together. A representative single-field projection system uses a 1:1 magnification lens for imaging the mask pattern onto the substrate, a 1-2 kW mercury-xenon arc lamp, a heat-filtering mirror, and a condenser to direct the radiation to the mask. For different resolution requirements, the maximum image field of the projection lens is different. For example, whereas a 1-mil resolution can be obtained over a 4-inch square field, a 1-micron resolution must be limited to a field diameter of 2-3 cm. Step-and-repeat systems use reduction imaging, typically in the range 2:1-10:1. Generally, systems with larger reduction ratios provide higher resolution, but also lower throughput.
Laser direct writing systems, which write the pattern on the substrate with a focused scanning beam, suffer from an inherently slow speed due to their bit-by-bit, serial mode of addressing. A representative direct writing system uses a focused blue or ultraviolet laser beam in a raster scanning fashion to expose each pixel on the substrate. The focused spot is moved across the substrate in one dimension, while the stage holding the substrate is translated in the orthogonal dimension. Due to serial addressing, the processing times for direct-write systems are long, ranging from a few minutes to a few hours per substrate, depending upon the resolution and complexity of the pattern data.
Thus, existing technologies for microelectronic patterning suffer from major shortcomings, including defect generation on substrate, mask wear, limitation of resolution or field size, and low throughput. Ideally, one desires the throughput of contact printers as well as the high resolution available from projection imaging and direct writing, without any of the disadvantages of the three technologies. There is thus a great need for developing a patterning system for fabrication of electronic products which combines three major performance attributes: high resolution, high processing throughput, and ability to handle large substrate sizes.
An imaging technology which achieves the above objectives has been described by this inventor in his U.S. Pat. No. 4,924,257, Scan and Repeat High Resolution Lithography System, issued May 8, 1990 (application Ser. No. 253,717, filed Oct. 5, 1988). The referenced patent discloses a patterning technology that uses synchronized mask and substrate stages which are moved in such a way that a large number of small image fields are joined together `seamlessly` without any nonuniformly exposed overlaps or unexposed gaps. This invention discloses a projection imaging system in which the objective of seamless exposure for large-area, high-resolution, high-throughput patterning is achieved using a single, integrated stage assembly for both mask and substrate.